Antibiotic or antimicrobial substances have long been used to inhibit the growth of bacteria or other microbes and to treat bacterial or microbial infections in humans, other animals, and in tissue culture. The use of antibiotics or antimicrobials in a treatment regimen, however, has the undesirable effect of selecting for bacteria or other microbes which are resistant to those antibiotics or antimicrobials which are administered or applied. As a result, treatment regimens can be adversely affected or, in some cases, rendered ineffective. This necessitates a continual search for new antibiotics and antimicrobials.
Of particular interest is the discovery of bacteria which express a multiple antibiotic resistance phenotype (Mar). This phenotype entails simultaneous resistance to a multiplicity of antibiotics which are unrelated in chemical structure. The appearance of such bacteria and infections by such bacteria greatly increase the difficulty of identifying effective antibiotics and treating infections in humans or other animals.
Multiple antibiotic resistance in bacteria is most commonly associated with the presence of plasmids and/or transposons which contain one or more resistance genes, each encoding a single antibiotic resistance phenotype. Multiple antibiotic resistance associated with the chromosome, however, has been reported in Klebsiella, Enterobacter, Serratia (Gutmann et al., J. Infect. Dis. 151:501-507, 1985), Neisseria (Johnson and Morse, Sex. Transm. Dis. 15:217-224, 1988), and Escherichia (George and Levy, J. Bacteriol.155:531-540, 1983).
Bacteria expressing a chromosomal multiple antibiotic resistance phenotype can be isolated by selecting bacteria with a single antibiotic and then screening for cross-resistance to structurally unrelated antibiotics. For example, George and Levy initially described a chromosomal multiple antibiotic resistance system which exists in Escherichia coli and which can be selected by a single drug, e.g., tetracycline or chloramphenicol (George and Levy, 1983). In addition to resistance to the selective agents, the Mar phenotype includes resistance to structurally unrelated agents, including nalidixic acid, rifampin, penicillins, and cephalosporins (George and Levy 1983) as well as fluoroquinolones (Cohen et al. 1989).
The chromosomal gene locus which correlates with the Mar phenotype observed in E. coli has been identified. The chromosomal mar locus, located at 34 min on the E. coli chromosomal map, is involved in the regulation of intrinsic susceptibility to structurally unrelated antibiotics (Cohen et al., J. Bacteriol. 175:1484-1492, 1993; Cohen et al., Antimicrob. Agents and Chemother. 33:1318-1325, 1989; Cohen et al., J. Bacteriol. 170:5416-22, 1988; Goldman et al., Antimicrob. Agents Chemother. 40:1266-1269, 1996), as well as the expression of antioxidant genes (Ariza et al., J. Bacteriol. 176:143-148, 1994; Greenberg et al., J. Bacteriol. 173:4433-4439, 1991) and internal pH homeostasis (Rosner and Slonczewski, J. Bacteriol. 170:5416-22, 1994). The mar locus consists of two transcription units (marC and marRAB) which are divergently transcribed from a central putative operator-promotor region (marO) (Cohen et al., 1993; Goldman et al., 1996). marR is the repressor of the marRAB operon (Cohen et al., 1993; Martin and Rosner, Proc. Natl. Acad. Sci. USA 92:5465-5460, 1995; Seoane and Levy, J. Bacteriol. 177:3414-3419, 1995). Mutations in marR result in increased expression of the marRAB operon. Overexpression of marA alone is sufficient to produce the multiple antibiotic resistance phenotype (Cohen et al., 1993; Gambino et al., J. Bacteriol. 175:2888-2894, 1993; Yan et al., Abstr. A-26, p. 5, In Abstracts of the 1992 General Meeting of the American Society for Microbiology, American Society for Microbiology, Washington, D.C., 1992). marB has no effect of its own; however, when it is present on the same construct with marA, it produces a small increase in antibiotic resistance (White et al., Abst A-104, p. 20. In Abstracts of the 1994 General Meeting of the American Society for Microbiology, American Society for Microbiology, Washington, D.C. 1994). The function of marC is unknown; however, it also appears to enhance the multiple antibiotic resistance phenotype when cloned on the same DNA fragment with the marRAB operon (Goldman et al., 1996; White et al., 1994).
Overexpression of marA confers multiple antibiotic resistance via increased efflux of antibiotics, including fluoroquinolones, tetracycline, and chloramphenicol (Cohen et al., 1989; George and Levy, 1983; McMurry et al., Antimicrob. Agents Chemother. 38:542-546, 1994). Transcription of the acrAB operon, which encodes a multi-drug efflux pump whose expression is modulated by global stress signals (Ma et al., Mol. Microbiol. 16:45-55, 1995; Ma et al., Mol. Microbiol. 19:101-112, 1996), was shown to be elevated in strains containing marR mutations and displaying the Mar phenotype (Okusu et al., J. Bacteriol. 178:306-308, 1996). Moreover inactivation of acrAB led to increased antibiotic susceptibility in wild type and Mar mutants (Okusu et al., 1996).
More recently, mutations of marR have been found in clinical isolates resistant to quinolones (Maneewannakul and Levy, 1996). Thus mar mutants can be selected under clinical conditions and not merely under controlled laboratory conditions. Early mar mutants (i.e.,"first-step" mar mutants) remain susceptible to many common antibiotics, although such mutants can achieve levels of clinical resistance to certain antibiotics, including tetracycline, nalidixic acid and rifampin (reviewed by Alekshun and Levy, Antimicrob. Agents Chemother. 41:2067-2075, 1997). First-step mar mutants thus may serve as precursors of bacterial mutants which display higher levels of resistance resulting from additional mutations on the chromosome. Thus it has been demonstrated that antibiotics can select for mutations in chromosomal gene loci which confer multiple antibiotic resistance under clinical conditions.
Non-antibiotic antibacterial compositions such as disinfectants are widely used in both clinical and consumer environments for reducing bacterial contamination of work surfaces, equipment, products and the like. These non-antibiotic antibacterial compositions have been incorporated into a wide spectrum of cleansers, disinfectant compositions, soaps, lotions, plastics, etc. It is not known whether exposure of bacteria to non-antibiotic antibacterial compositions also can select for bacterial mutants, including those which display a multiple antibiotic resistance phenotype.